The traditional method for plating tin-lead alloys, commonly called Solder Plate, is to have these two metals present in solution as fluoborates with an excess of fluoboric acid and boric acid. It is common to add various types of proteins, such as peptones, glue, gelatin, or meat proteins to obtain smoother deposits. The fluoborate-based bath containing peptone is most commonly used and is commercially successful in obtaining smooth matte deposits. Brighter tin-lead solder deposits can be obtained from these fluoborate-based baths by adding various brightening agents, such as aromatic alkane pyridine compounds, ethylene oxide wetting agents and formaldehyde.
Alkane or alkyl sulfonic acids containing 1 to 5 carbon atoms have also been used in electroplating solutions. One early example is U.S. Pat. No. 2,525,942 to Proell. Several more recent patents describe the use of various additives to improve the brightness of deposits, the useful current density ranges, and/or the solderability of the deposits and to improve the performance of the electroplating bath. Examples are U.S. Pat. Nos. 4,565,609, 4,701,244, and 5,066,367, all to Nobel et al.
Low-friction coatings have been suggested for use in electronic connectors. For example, an article by G. N. K. Ramesh Bapu, et al. ("Bapu"), entitled "Electrodeposition of Nickel-Polytetrafluoroethylene (PTFE) Polymer Composites," discloses use of a nickel-PTFE composite. The Bapu composites were obtained using a Watts nickel bath containing 25 ml/L PTFE suspension. The volume percent of PTFE in the composite increased with PTFE concentration in the bath and the current density, and resulted in an adherent, smooth, uniform and semi-bright deposit when the bath was operated at 6.0 A/dm.sup.2 at a pH of 3 and 50.degree. C.
An article by V. Bhalla et al. ("Bhalla"), entitled "Friction and Wear Characteristics of Electrodeposited Copper Composites," discloses low friction where PTFE particles are dispersoids in copper composite coatings. Bhalla found that, although the Cu-PTFE composite showed the lowest wear index, at best the Cu-PTFE had a higher coefficient of friction than copper-graphite when deposited at 2 A/dm.sup.2 and 30.degree. C. This reference concludes that a copper-graphite composite was the best self-lubricating coating of those tested.
However, conventional tin, lead, and tin-lead alloys are a favored electrofinish for corrosion protection of electronic connectors, including the crimp and contact portions of the connectors. The main problem with these conventional electroplating materials is that the coefficient of friction for tin, lead, and tin-lead alloys is typically greater than 0.8.
The high coefficient of friction seen in these conventional tin or lead alloys results in a high insertion force required to connect separable connectors, as well as fretting corrosion of the connectors. This high insertion force requires larger and bulkier connectors to be manufactured, thereby increasing the size of electronic components. The high insertion force also leads to high rates of breakage when assembling connectors. Thus, electroplating materials having a reduced coefficient of friction would be desirable to provide a reduced insertion force when assembling connectors. This would advantageously permit further miniaturization of electronic components using reduced friction connectors, and would minimize fretting corrosion of separable connectors.